```
class Solution {
public:
vector < vector<int> > res;
void dfs(int n,int k,int cnt,int st,vector<int>&tmp){
if (cnt==k) {
res.push_back(tmp);
return;
}
for (int i = st; i<=n; i++){
tmp[cnt] = i;
dfs(n,k,cnt+1,i+1,tmp);
}
}
vector< vector<int> > combine(int n, int k) {
vector <int> tmp(k,0);
dfs(n,k,0,1,tmp);
return res;
}
};
```